Meta offers several sources: website visitors tracked by the Pixel and Conversions API, customer lists you upload (hashed before matching), app activity, and engagement with your Facebook Page, Instagram profile, videos, lead forms, events, and more. Each can be filtered by recency and behavior — for example, people who viewed a pricing page in the last 14 days.
Custom Audiences power retargeting, exclusions, and lookalikes, and they're increasingly used as suggestions within Advantage+ audience rather than as hard targeting. Because many sources depend on tracking and consent, audience sizes can shrink with privacy changes — another reason first-party customer lists are so valuable.
Example
A contractor builds a Custom Audience of people who watched at least half of its project video and retargets them with a free-estimate offer.
How Sloss Digital uses it
We build Custom Audiences by intent and recency for retargeting, use customer lists for exclusions and seeding, and refresh them automatically from the CRM.